The present invention relates generally to current sensing devices for electrical systems, and more particularly to timed reset fault indicators for alternating current power systems.
Various types of self-powered fault indicators have been constructed for detecting electrical faults in power distribution systems, including clamp-on type fault indicators, which clamp directly over cables in the systems and derive their operating power from inductive and/or capacitive coupling to the monitored conductor; and test point type fault indicators, which are mounted over test points on cables or associated connectors of the systems and derive their operating power from capacitive coupling to the monitored conductor.
Such fault indicators may be either of the manually resetting type, wherein it is necessary that the indicators be physically reset, or of the self-resetting type, wherein the indicators are reset upon restoration of line current. Examples of such fault indicators are found in products manufactured by E. O. Schweitzer Manufacturing Company of Mundelein, Ill., and in U.S. Pat. Nos. 3,676,740, 3,906,477, 4,063,171, 4,234,847, 4,375,617, 4,438,403, 4,456,873, 4,458,198, 4,495,489, 4,974,329, 5,677,678, 6,016,105, 6,133,723 and 6,133,724.
Detection of fault currents in a monitored conductor by a fault indicator is typically accomplished by magnetic switch means, such as a magnetic reed switch, in close proximity to the conductor being monitored. Upon occurrence of an abnormally high fault-associated magnetic field around the conductor, the magnetic switch actuates a trip circuit that produces current flow in a trip winding to position an indicator flag visible from the exterior of the indicator to a trip or fault indicating position. Upon restoration of current in the conductor, a reset circuit is actuated to produce current flow in a reset winding to reposition the target indicator to a reset or non-fault indicating position, or the fault indicator may be manually reset.
Some prior art fault indicators utilize light emitting diodes (LEDs) to display a fault condition. However, activation of LEDs requires a source of power greater than that typically obtainable from inductive or capacitive coupling to a monitored conductor, such as from an internal battery. Even if the LEDs are controlled to flash intermittently, the intermittent current drain from the internal battery is not insubstantial, and replacement of the battery is sometimes required. There is therefore a need to operate the LEDs at reduced current levels especially at nighttime.
There is therefore a need for a battery-powered fault indicator with an energy conservation mode in which there is insubstantial current drain from a high capacity battery, such that the battery may never need replacement. There is also a need for a battery-powered fault indicator with circuitry, including a microprocessor, which places insubstantial current drain on the battery until a fault is detected. There is a further need for such a fault indicator that returns to the energy conservation mode when the fault condition is corrected or when the fault indicator is reset.
In certain other applications, the need arises for a fault indicator that will continue to display a prior fault condition for a predetermined amount of time, such as in the range of one hour to twenty-four hours, rather than self-resetting upon restoration of current in the conductor. Such timed reset fault indicators should be capable of self-resetting after termination of the predetermined time.
Some of these applications also require voltage in-rush restraint and/or current in-rush restraint to prevent false tripping due to voltage and/or current surges, such as when a reclosing relay of a power distribution system closes.
Because of the compact construction and limited power available in self-powered fault indicators, it is preferable that the desired functions of the fault indicator be accomplished with minimal structure and with internal circuitry that has minimal current drain on a high capacity battery. The fault indicator must also provide highly reliable and extended operation over a number of years.
Accordingly, it is a general object of the present invention to provide a new and improved fault indicator with internal circuitry having insubstantial current drain on the battery during non-fault conditions such that the battery may never need replacement during the expected lifetime of the fault indicator.
Another object of the present invention is to provide a fault indicator that is microprocessor-controlled, with the microprocessor operating in a sleep mode during non-fault conditions to conserve battery life.
Yet another object of the present invention is to provide a fault indicator with highly visible LED indicators that are periodically illuminated for a predetermined amount of time after sensing a fault on a monitored conductor.
A further object of the present invention is to sense the ambient lighting conditions and to reduce the current supplied to the LEDs under lower ambient light levels, such as at night, to further reduce current drain on the battery.
A still further object of the present invention is to provide inrush restraint for such a fault indicator to avoid false tripping on voltage surges in the electrical distribution system, such as those surges that are typically encountered when a reclosing relay applies power to the system.
Another object of the present invention is to provide an inrush restraint capability in a fault indicator can satisfy the needs of operating over a wide range of line voltages.
Yet another object of the present invention is to provide an improved electrostatic pickup for coupling to the electromagnetic field associated with a conductor to supply power to the inrush restraint circuitry and to provide less susceptibility to surges on adjacent conductors.